Wave soldering is a well-known method by which solder connections are formed on a printed circuit board (PCB) by bringing the circuit board in contact with a wave of molten solder flowing upward from a nozzle. Wave soldering methods typically apply a relatively thin coating of solder, and therefore find widest use for applications in which a solderable contact or metal lead is to be coated with solder to form a solder fillet or solder connection, respectively. One such example is a circuit component having leads that extend through a circuit board, and to which solder is applied to physically and electrically connect the component to the board. During wave soldering, the circuit board, supported at its perimeter with a pallet, passes through molten solder coming up from the nozzle, such that the ends of the leads projecting through the circuit board are brought in contact with the upward-flowing molten solder. In the process of adhering to the leads, the solder forms solder connections or fillets.
While leaded circuit devices are widely employed in electronic applications, and wave soldering is widely practiced to attach and electrically connect such devices to printed circuit boards, a continuous effort to reduce the size of circuit board assemblies has promoted the use of such advanced packaging technologies as tape automated bonding (TAB), chip on board (COB), flip chips, multichip modules (MCM) and ball grid arrays (BGA). These devices are generally surface-mount technology (SMT) components attached to a circuit board with solder, such as solder bumps that are formed on the devices and then reflowed to solder the devices to appropriate conductor patterns on the board.
While highly successful, SMT devices are vulnerable to downstream processes that can damage their solder connections. As an example, because of their rigidity, flip chips are vulnerable to mechanical loads that distort the circuit board, causing fracturing or separation of their solder bump connections. Wave soldering is a notable example of such downstream processing, during which thermally-induced board distortion occurs as a result of molten solder contacting only one surface of the board. If not minimized, this distortion may cause significant quality concerns, including fracturing and/or separation of solder connections, potential destruction of the affected device(s), and reduced overall system reliability. In the past, PCBs have been of sufficient thickness to allow wave soldering without unacceptable levels of board distortion. Preheating PCBs to achieve a more uniform temperature throughout the board has been used to further reduce board distortion, to the extent that an adequate board flatness of about 0.020 to 0.030 inch (about 0.50 to 0.75 mm) has typically been maintained. However, thin laminate PCBs (e.g., thicknesses of 0.032 inch (about 0.8 mm) and less) are more prone to distortion during wave soldering, with the result that fracturing and separation of flip chip solder connections is much more likely to occur. While the incidence of solder connection fracturing and separation can be reduced or avoided by the use of alternative circuit devices and soldering processes, such solutions are often not practical or cost effective.
Therefore, what is needed is a method for reducing the incidence of solder connection fracturing and separation of SMT devices during a wave soldering operation.